net/mlx5: SD, Add debugfs

Add debugfs entries that describe the Socket-Direct group.

Example:
$ grep -H . /sys/kernel/debug/mlx5/0000\:08\:00.0/multi-pf/*
/sys/kernel/debug/mlx5/0000:08:00.0/multi-pf/group_id:0x00000101
/sys/kernel/debug/mlx5/0000:08:00.0/multi-pf/primary:0000:08:00.0 vhca 0x0
/sys/kernel/debug/mlx5/0000:08:00.0/multi-pf/secondary_0:0000:09:00.0 vhca 0x2

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>